build: Only include enum header in libst dependency

Compiling the generated source for each consumer of the dependency
means we end up trying to register the enum types multiple times,
resulting in a fatal failure on startup. Luckily code outside libst
itself only depends on the header, which doesn't cause those issues.

src/st/meson.build

@@ -128,7 +128,7 @@ libst = shared_library('st-1.0',
 )
 
 libst_dep = declare_dependency(link_with: libst,
-  sources: st_enums
+  sources: st_enums[1]
 )
 
 test_theme = executable('test-theme',